The present invention relates to an improved oriented multi-layer polymer film with high opacity, whiteness, and strength that is economically made.
More specifically, the invention relates to an oriented multi-layer polymer film which includes a core made of a polyolefin matrix material, at least one opaque thermoplastic polymer layer on at least one side of the core, and at least one thermoplastic polymer skin layer on the opaque layer.
Oriented multi-layer films of the present invention are widely used in packaging of certain types of foods, such as snack foods like potato chips, cookies and the like. A desirable property in such a packaging film is opacity which protects the packaged product from deterioration caused by exposure to light. In particular, it has been found that certain wavelengths of light, up to about 450 nm cause increased spoilage in such packaged products. In addition, the consumer appears to associate opacity or white coloration with a sanitary product and such white coloration masks oil and grease from the baked item.
Oriented multi-layer opaque films contain at least one layer with pigments or vacuole-inducing particles or a combination thereof, causing the film to have significantly reduced light transmission compared with transparent films. For the purposes of the present invention, xe2x80x9copaque filmxe2x80x9d means a nontransparent film whose light transparency, measured in accordance with the ASTM D 1003-77, is at most 80%.
Other oriented opaque polymer film compositions are known in the art. U.S. Pat. No. 4,377,616 discloses an opaque biaxially oriented polymer film structure which includes a thermoplastic polymer matrix core possessing numerous voids, a substantial number of which contain at least one spherical void-initiating particle, and transparent thermoplastic skin layers adhering to the surfaces of the core. The unique structure of the core imparts a much higher degree of opacity, possibly due to the effects of light scattering, than that possible from the use of an opacifying pigment alone. However, the core of this film contains voids which weaken the film structure and seal seam strength.
Similarly, U.S. Pat. No. 4,632,869 discloses an opaque, biaxially oriented film structure with a polymer matrix containing a strata of voids in the core. The structure may also include thermoplastic skin layers and from about 1% to 3% by weight of titanium dioxide (TiO2) or other colored oxides for color. The film has weakened strength due to voids in the core.
U.S. Pat. No. 4,741,950 discloses a differential opaque polymer film with a core layer containing numerous microscopic voids, a rough-appearing wettable first skin layer which contains an antiblocking agent such as silica, silicate, clay, diatomaceous earth, talc and glass, and a second wettable skin layer with a smooth appearance which may be metallized. Titanium dioxide may be present in the core. Among other shortcomings, this patent uses an opacifying pigment in the core which leads to contamination of equipment used in the printing of the film. When the equipment used in the manufacturing process is contaminated, great expense in the cleaning process is incurred.
Similar contamination occurs when the opacifying agent is disposed in the outer layer of the film. For example, U.S. Pat. No. 5,425,996 places the opacifying pigment in the outer layer of opaque polymer films. This design also leads to contamination of equipment used in the printing of film.
Thickness of the film is also important for film structure and seal seam strength. U.S. Pat. No. 5,716,698 discloses a peelable, oriented, opaque, multi layer polyolefin film having at least one opaque layer. All layers of the film are essentially vacuole-free for easy pealability that does not destroy the film.
U.S. Pat. No. 5,443,915 discloses an oriented, polyolefin film with a white-opaque cold seal receptive skin layer on one side of the core and a vacuumed deposited metal layer on the other side of the core. The cold receptive polyolefin layer contains a slip agent and 10% to 40% by weight of titanium dioxide. The core is substantially free of opacifying fillers and voids. One disadvantage of this invention is that the white-opaque cold seal receptive skin layer is not designed for heat sealing. The thickness of the film is about 0.2 mil. which lends itself to poor machinability and film strength.
Despite these advances in the art, a need still exists for an oriented multi-layer opaque polymer film that can economically be made with high opacity that provides improved processing characteristics such as heat sealing and machinability. There is a need for opaque polymer film that provides high whiteness, strength and stiffness that also does not contaminate equipment used in laminating and/or packaging of films.
The present invention, which addresses the needs of the prior art, is directed to an oriented multi-layer polymer film including a core made of a thermoplastic polyolefin matrix material. This core is substantially free of cavitations, voids and fillers. The oriented multi-layer polymer film of the present invention has at least one opaque thermoplastic polymer layer on at least one side of the core. This opaque thermoplastic polymer layer contains an opacifying pigment. The oriented multi-layer polymer film of the present invention has a thickness from about 0.45 mil. (gauge) to about 2 mil. (gauge) and at least one thermoplastic polymer skin layer on an outermost side of at least one opaque layer or the core. Accordingly, the present invention provides a film that does not contaminate equipment used in laminating and/or packaging of polymer films.
In addition, the polyolefin material used in the thermoplastic polyolefin matrix, as well as the opaque thermoplastic polymer layer, and the thermoplastic polymer skin layer can be materials and mixtures of olefinic homo-, co-, or terpolymers of propylene, polypropylene, isotactic polypropylene, high density polyethylene (HDPE), high density polypropylene, linear low density polyethylene, propylene, butylene, ethylene, and butene.
In one exemplary embodiment, the oriented multi-layer polymer film includes a core with two layers of polypropylene (PP), and an opaque thermoplastic polymer layer of polypropylene (PP) and titanium dioxide. Titanium dioxide is present in an amount from about 0.5% to about 40% by weight based on the weight of the opaque thermoplastic polymer layer. In addition, the thermoplastic polymer skin layer includes high density polyethylene (HDPE) and a second thermoplastic polymer skin layer of ethylene-propylene-butylene (EPB) terpolymer on at least one side of the opaque thermoplastic polymer skin layer.
The oriented multi-layer polymer film includes at least one heat sealable thermoplastic polymer skin layer outermost on one or both sides of the film. An advantage to this structure is that the film can easily be heat sealed.
As a result of the present invention, a significant white-opaque appearance is provided when titanium dioxide is used as the opacifying pigment in the opaque thermoplastic layer, up to not greater than about 40% by weight based on the weight of the opaque thermoplastic polymer skin layer. Such white coloration masks oil and grease from the baked item. This white color is enhanced when at least one thermoplastic polymer skin layer is metallized, thereby eliminating the need for white color printing of the film.
A preferred embodiment of the oriented multi-layer polymer film includes a core of polypropylene (PP), an opaque thermoplastic polymer layer of polypropylene and titanium dioxide on each side of the core. The titanium dioxide is present in an amount from about 5% to about 18% by weight based on the weight of the opaque thermoplastic polymer skin layer. A thermoplastic polymer skin layer of high density polyethylene (HDPE) on at least one side of the opaque thermoplastic polymer layer and a second thermoplastic polymer skin layer of ethylene-propylene-butylene (EPB) on at least one side of the opaque thermoplastic polymer layer. These additional layers allow for a lower amount of opacifying pigment to be used in the oriented multi-layer polymer film. Accordingly, a more economical oriented multi-layer polymer film can be produced.
Additionally, the oriented multi-layer polymer film of the present invention contains an anti-blocking agent and an anti-static agent which allows easy machinability of the film.
Other improvements which the present invention provides over the prior art will be identified as a result of the following description which sets forth the preferred embodiments of the present invention. The description is not in any way intended to limit the scope of the present invention, but rather only to provide a working example of the present preferred embodiments. The scope of the present invention will be pointed out in the appended claims.